1. Field of the Invention
The invention relates to an optical disk including a substrate, a recording layer formed on one side of the substrate, and a cover sheet coated over the recording layer, as well as to a method for affixing a display sheet on the optical disk. The invention relates to a disk for recording a large volume of information, and more particularly, to a disk which performs recording operation through use of a UV ray of shorter wavelength, such as a Blu-Ray (blue ray) disk.
2. Description of the Related Art
Development of an optical disk for realizing large-capacity recording has been pursued with a view toward commercialization of an optical disk to be used for performing recording and playback operation through use of a high power violet laser light source. As shown in FIG. 10, a DVD is constructed by laminating together two disk substrates, each having a thickness of 0.6 mm. In contrast, an optical disk 1 adopts a so-called “0.1 mm cover layer system,” wherein a recording surface 5 formed on a substrate 3 is covered with a thin cover layer 7 of 0.1 mm thickness. The reason why the cover layer 7 is made thin to a thickness of 0.1 mm is that a reduction in thickness is indispensable for reducing the diameter of a laser beam spot to about one-fifth the diameter of a laser beam spot adopted by a current DVD.
Specifically, if the wavelength of the light source is shortened from 650 nm or thereabouts used for a related-art DVD to 405 nm or thereabouts and a numerical aperture (NA) is increased from 0.6 to 0.85, the diameter of the beam spot can be reduced. However, the higher the NA, the smaller the allowable range of inclination of the optical axis (tilt margin) with reference to the recording surface. For this reason, expansion of the tilt margin to the same extent as that of the current DVD is enabled by reducing the thickness of the cover layer 7. As a result, warpage and inclination of the optical disk 1 or an error in assembly operation performed by a system can be addressed to the same level as that achieved by the DVD (see “Nikkei Electronics,” pp. 79-86, Mar. 11, 2002.).
The cover layer 7 of the optical disk is usually coated with UV hardening resin and then exposed to UV rays for hardening. Organic-pigment-based recording material is used for the recording layer 5. Hence, an UV hardening method for use in hardening the cover layer 7 is not preferable. For this reason, affixing of a thin-layer plastic sheet (polycarbonate, TAC, PET, or the like) on the recording layer 5 as a cover sheet is conceivable. According to the thin-layer plastic sheet affixing method, an appropriate thickness of the cover layer 7 is obtained, and no damage is inflicted on the organic-pigment-based recording material, which would other wise be caused when UV hardening is performed. Therefore, the thin-layer plastic sheet affixing method can be said to be a preferable cover layer formation method. However, as shown in FIG. 11, a plastic sheet 9 is additionally subjected to an extension process during the manufacturing process, for ensuring strength and the accuracy of thickness. Consequently, the plastic sheet 9 possesses a thermal contraction characteristic (Bowing phenomenon), wherein a given directionality is imparted to the direction of heat contraction (see the directions indicated by the arrows provided in circles shown in FIG. 11). When the plastic sheet 9 is applied integrally over the substrate 3 as a cover sheet 11 of the recording layer 5 as shown in FIG. 12, no problem arises when the optical disk is stored in an ordinary environment. However, when an optical disk is likely to be stored at high temperature and high humidity (e.g., when an optical disk is placed in a car), the cover sheet 11, which causes heat contraction, pulls the substrate 3 in the same manner as does bimetal. There arises a risk of warpage in a disk substrate 13. If such warpage arises in the disk substrate 13, a tilt change α which arises at the time of radiation of a laser becomes greater as shown in FIG. 13, thereby impairing the appearance of the optical disk as well as deteriorating read/write characteristics of the same.
In JP-A-2002-92965, there has also already been proposed an optical disk, wherein an ultra-thin transparent cover layer is provided on a substrate with information signals formed therein, and wherein the transparent-cover-side of the substrate is exposed to a laser beam, to thereby record/reproduce the information signals. Since an optical disk substrate can usually be formed to a thickness of about 1.2 mm, the optical disk enables accurate transfer of irregularities even when the disk substrate is manufactured by means of an injection molding method. The laser beam used for recording/reproduction is radiated onto the side of the substrate covered with the ultra-thin transparent cover layer, and hence sufficient addressing of an increase in recording capacity has come to be feasible. As mentioned above, the Blu-Ray disk requires a large-diameter anti-reflection characteristic, and hence a recording surface of the substrate is coated with a ultra-thin transparent cover layer, to thereby stabilize a characteristic of the disk and maintain the appearance of the disk when in use.
In this case, the cover layer of the Blu-Ray disk has a thickness of about 0.1 mm. After the disk has been coated with UV curing resin, the disk is usually subjected to hardening treatment. Although various Blu-Ray recording methods have already been proposed, users expect rewritability or the like.
In order to ensure rewritability, an organic-pigment-based recording material is frequently used. Although a UV laser beam is used for recording operation, application of the UV-ray curing method to curing of a cover layer is less desirable as mentioned above. As a result of a thin-layer plastic sheet (polycarbonate, TAC, PET, or the like) being affixed to the disk, an appropriate cover thickness is obtained. Further, UV curing is not performed, and hence damage is less frequently inflicted on the organic-pigment-based recording material. Therefore, affixing of a thin-layer plastic sheet can be said to be an appropriate method.
In order to ensure strength and accuracy of thickness, a plastic sheet is inevitably subjected to a pull process during manufacturing processes. Consequently, the plastic sheet is imparted with an irreversible thermal expansion/shrinkage characteristic. When the material is integrally affixed to the disk substrate, no particular problem is encountered in preserving the disk in a normal environment. There may arise a case where the dimension of a cover layer is changed by thermal shrinkage or moisture shrinkage resulting from the disk being preserved at high temperature and high humidity, thereby imparting “warpage” to the substrate with the cover affixed thereto and inducing a change in the amount of tilt to exert an adverse influence on a recording/reproduction characteristic.
A rigorous anti-reflection (AR) characteristic is set for the Blu-Ray disk [hence, the influence of the AR characteristic is greater than that on a CD or a DVD], and hence a nominal change in tilt induces deterioration of a characteristic.